Method of removing scale from ferrous articles prior to coating



United States Patent 3,440,082 METHOD OF REMOVING SCALE FROM FERROUS ARTICLES PRIOR TO COATING Leonard J. Kube, Milwaukee, Wis., assignor to A. O. Smith Corporation, Milwaukee, Wis., a corporation of New York No Drawing. Filed Mar. 29, 1965, Ser. No. 443,649 Int. Cl. B44d 1/092, 1/34; C23c 7/00 US. Cl. 117-49 4 Claims ABSTRACT OF THE DISCLOSURE A method of removing scale from ferrous articles by blasting the article with finely divided metallic particles while the article is at an elevated temperature in the range of 300- 1,000 F. and exposed to the atmosphere. After blasting, the article is cooled in the atmosphere to room temperature thereby providing a treated surface conditioned for the application of a protective coating such as paint.

This invention relates to a process of treating metals and more particularly to a process for removing oxide scale from ferrous metals.

The conventional practice in the industry for removing oxide scale from ferrous metals is to grit or shot blast the article at room temperature. Blasting not only removes the oxide scale to prepare the surface for the application of protective coatings, but provides a surface treatment which is beneficial from a fatigue resistance standpoint.

In the fabrication of channel-shaped vehicle side rails, the side rails are normally tempered at an elevated temperature, cooled to room temperature, shot blasted and subsequently painted. In some cases, all of the scale cannot be completely removed by a single shot blasting treatment so that the side rail must be run through the shot blasting zone several times to completely remove the oxide scale. This increases the overall cost of production and tends to disrupt a continuous production run.

The present invention is based on the discovery that oxide scale can be more easily removed from ferrous articles by blasting the article while the article is at an elevated temperature in the range of 300 to 1,000 F. For a reason not fully understood, the blasting at this elevated temperature improves the rate of scale removal and produces a better surface condition for adherence of a protective coating, such as paint or the like.

The process of the invention is particularly adaptable to carbon steel articles or other ferrous materials which tend to corrode and develop an oxide film or scale. In

carrying out the process of the invention the ferrous article is initially heated to a temperature in the range of 300 to 1,000 P. and blasted with either grit or shot while at this elevated temperature. Depending on the type of article and the nature of heat treatment, if any, to which the article is to be subjected, the article can either be heated from room temperature to the range of 300 to 1,000 E, or alternately it can be cooled from a higher heat treating temperature to a temperature in the range of 300 to l,000 F. In either case, the article is blasted while at the temperature of 300 to 1,000 F. and preferably 500 to 700 F.

In the blasting operation, either shot or grit can be used. The particular size of the shot or grit is not critical and any conventional sized shot or grit can be employed. For most operations the particle size will be in the range of 0.05 to 0.06 inch in diameter.

After blasting, the article is cooled in air to room temperature and a protective coating, such as paint, lacquer,

varnish, plastic, silicate or the like, can then be applied over the hot blasted surface.

-It has been found that by blasting the ferrous article at an elevated temperature, the oxide scale is more easily removed. In addition, blasting at this elevated temperature improves the surface condition of the article for adherence of the protective coating.

A specific example of the process of the invention is as follows:

Three A" x 4" x 8" carbon steel panels were prepared. One side of Panel A was shot blasted at room temperature while one side of Panel B was shot blasted at a temperature of 600 F. One side of Panel C was shot blasted at a temperature of '800" F., and subsequently Panel C was cooled to room temperature and the opposite surface of Panel C was shot blasted While the panel was at this temperature. The time required for scale removal on the one surface of Panel C at 800 F. was approximately one-half the time required for scale removal on the opposite surface at room temperature.

A coat of Mil TIP-636 side bar primer was then applied by spraying to the blasted surfaces of Panels A and B as well as both blasted surfaces of Panel C.

The panels were then placed in a salt spray bath using a 5% salt solution and a temperature of F.

After 20 hours of exposure, the paint on the cold blasted surfaces showed failure and evidence of corrosion of the base metal was visible. The paint on the hot blasted surfaces of Panels B and C did not show failure and evidence of corrosion until 46 hours.

The results of these tests clearly evidence the improved adherence of the protective coating which is brought about by shot blasting at an elevated temperature.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A method of treating ferrous articles to remove scale and provide an improved surface condition comprising the steps of heating the article to a temperature in the range of 300 to 1,000 F., blasting finely divided metal particles against the article while the article is at said temperature to remove the scale, and subsequently cooling the article in air to room temperature.

2. A method of treating ferrous articles to remove scale and provide an improved surface condition comprising the steps of heating the article to a temperature in the range of 300 to 1,000 E, shot blasting a surface of the article while the article is at said temperature and exposed to the atmosphere to remove the scale from the said surface, cooling the article while exposed to the atmosphere to room temperature, and subsequently applying a protective coating to the shot blasted surface.

3. A method of treating a carbon steel article, comprising the steps of heating the article to an elevated tempering temperature, cooling said article from said temperature, blasting finely divided metallic particles against said article as the article is being cooled and when said article is at a second temperature lower than said tempering temperature and in the range of 300 to 1,000 F. to remove the oxide scale from said article and cooling 5e article in air to room temperature.

4. A method of treating a carbon steel article, comprising the steps of heating the article to an elevated tempering temperature, cooling said article in air from said temperature to a second lower temperature in the range of 300 to l,000 F., blasting finely divided metallic particles against said article when said article is at said lower temperature and exposed to air to remove the oxide scale from said article, cooling the article in air to room temperature, and subsequently applying a coating of paint to said article.

References Cited UNITED STATES PATENTS Burns, R. M. et al., Protective Coatings for Metals, 2nd ed., N.Y., Reinhold, 1955, pp. 34-35.

5 ALFRED L. LEAVITT, Primary Examiner.

Mathias 117 49 X J. R. BATTEN, Assistant Examiner. Broadfield 51-320 Baer et a1 117 50 CL Dill 51320 10 51319; 117-132, 135.1 

